YELLOWSTONE NATIONAL PARK — What makes the rocks in Yellowstone National Park (YNP) so colorful? This week’s Caldera Chronicles explores why Yellowstone is called Yellowstone, and the minerals and chemical elements that make up the internal structure of the rocks.
Research Hydrologist Shaul Hurwitz with the U.S. Geological Survey at the Yellowstone Volcano Observatory begins by explaining the origins of the name of the park. YNP was named after the Yellowstone River, which runs through the park and is lined with yellow sandstones in eastern Montana.
In the indigenous Minnetaree language, it was called Mi tse a-da-zi, which literally translates to “Rock Yellow River.”
Rocks like the yellow sandstones in YNP get their color from a variety of sources, but usually it’s the minerals and chemicals present in the stones.
“When sunlight hits a rock, the atoms and trace metals in some of its minerals absorb certain wavelengths of the light and reflect others,” Hurwitz writes. “These reflected wavelengths determine the color that you see. The visible spectrum is the small segment of the electromagnetic spectrum that the human eye can see, ranging from roughly 380 to 750 nanometers in wavelength. Some colors such as magenta, purple, or pink result from a mixture of multiple wavelengths.”
Hurwitz used diamonds as an example. Pure diamond is a mineral that consists of only carbon, but other colors of diamonds — yellow, green and blue, for example — have low concentrations of other elements, such as nitrogen, sulfur and boron.
Other colorful rocks of note in the west are the red sandstone sedimentary rocks, which have high levels of some types of iron oxides that give them their red color. Many of the sandstones in the Grand Canyon, Arizona, or Canyonlands National Park, Utah, are red for this reason.
Similarly here, the sandstone lining the Yellowstone River appear yellow because of iron oxides and minerals like quartz and feldspar. The amounts of iron oxides can affect the tone of the yellow. Higher concentrations result in more saturated golden colors.
It turns out that the color of the rocks surrounding a place can provide a lot of information about the minerals present, and the conditions in which they were formed. The west is not only wild, but also quite colorful.
